Transistors formed over a glass substrate or the like are manufactured using amorphous silicon, polycrystalline silicon, or the like, as typically seen in liquid crystal display devices. Although transistors manufactured using amorphous silicon have low field-effect mobility, they can be manufactured over a larger glass substrate. On the other hand, although transistors manufactured using polycrystalline silicon have high field-effect mobility, they are not suitable for being manufactured over a larger glass substrate.
In contrast to transistors manufactured using silicon, attention has been drawn to a technique by which a transistor is manufactured using an oxide semiconductor and applied to an electronic device or an optical device. For example, Patent Document 1 and Patent Document 2 disclose a technique by which a transistor is manufactured using zinc oxide or an In—Ga—Zn—O-based oxide as an oxide semiconductor and is used as a switching element of a pixel or the like of a display device.
Patent Document 3 discloses a technique by which, in a staggered transistor including an oxide semiconductor, a highly conductive oxide semiconductor containing nitrogen is provided as buffer layers between a source region and a source electrode and between a drain region and a drain electrode, and thereby contact resistance between the oxide semiconductor and the source electrode and between the oxide semiconductor and the drain electrode is reduced.
Non-Patent Document 1 discloses a top-gate oxide semiconductor transistor in which a channel region, a source region, and a drain region are formed in a self-aligned manner.